1. Field
The invention is directed to the use of a liquid crystal composition suitable for a bistable liquid crystal device, and specially a bistable liquid crystal device with anchoring breaking, and a bistable liquid crystal device with anchoring breaking comprising the liquid crystal composition.
2. Background
Liquid crystal display (LCD) devices are increasingly used in display applications for which there are constraints on the volume, weight or electricity consumption. Therefore, they are found in all sorts of mobile applications, for example laptop computers, electronic books, personal assistants and mobile phones.
1) Practical Definition of Azimuthal and Zenithal Anchoring
The anchoring and anchoring breaking concepts for liquid crystal molecules on surfaces are highly technical, and they can be defined. The orientation of the liquid crystal molecules by surfaces is called anchoring. The source of anchoring is anisotropy of the interaction between the liquid crystal and the surface. Anchoring can be characterised by its efficiency and by the preferential direction imposed by the surface on adjacent liquid crystal molecules. This direction, called the easy axis, is defined by the azimuth angle φ0 and the zenithal angle θ0, (see FIG. 1). The nematic director, in other words the average direction of liquid crystal molecules, is drawn towards the easy axis through the surface. If there is no external influence, the liquid crystal director is oriented parallel to the easy axis to minimise the interaction energy with the surface. This energy (anchoring energy) may be written as follows as a first approximation (A. Rapini and M. Papoular, J. Phys. (Fr) C4, 30, 54-56 (1969)):
                              g          ⁡                      (                          θ              ,              φ                        )                          =                                                            W                z                            2                        ⁢                                          sin                2                            ⁡                              (                                  θ                  -                                      θ                    0                                                  )                                              +                                                    W                a                            2                        ⁢                                          sin                2                            ⁡                              (                                  φ                  -                                      φ                    0                                                  )                                                                        (        1        )            
where θ and φ are the zenithal angle and the azimuth angle of the nematic director on the surface, Wz and Wa are the surface densities of the zenithal and azimuth anchoring energies. To simplify the description, we will call them anchoring energies. On most solid surfaces, the zenithal anchoring energy is one or two orders of magnitude higher than the azimuthal anchoring energy. The azimuthal anchoring energy depends mainly on anisotropy induced on the surface by treatments, and in this case the nature of the materials is not as important
If the orientation of the director in the volume is different from the direction of the easy axis, the texture is deformed. The surface energy is no longer zero and the result is also a volume energy, characterised by an elasticity factor K that depends on the deformation. The surface energy can be characterised by its extrapolation length that is the ratio between the bulk elasticity factor and the anchoring energy.
The extrapolation length of the azimuthal anchoring is denoted Laz=K22/Waz. In practice, a azimuthal anchoring is considered as being strong if Laz<150 nm.
The extrapolation length of the zenithal anchoring is denoted Lz=K33/Wz. In practice, a zenithal anchoring is considered as being strong if Lz<15 nm and weak if Lz>25 mm.
Azimuthal and zenithal values are given in EP0859970 and U.S. Pat. No. 6,237,017, which are herein incorporated by reference.
2) Anchoring of Molecules in “Classical” Displays
In their simplest form, electrically controlled display devices comprise a liquid crystal material confined between two plates, at least one of which is transparent. Each of these plates is provided with an electrode on its internal face and was subjected to a surface treatment that orients the liquid crystal. It is said that this treatment anchors the molecules on the walls of the cell. By applying a potential difference exceeding a voltage called the Fréedericksz threshold voltage between the electrodes of the two plates, the orientation of the liquid crystal varies under the action of the electric field. Due optical anisotropy of the liquid crystal, these orientation variations change the optical properties of the display depending on the amplitude of the applied field.
All these displays called “classical” displays have a common characteristic; when the external electric field is switched off, the displayed information disappears more of less quickly. The anchoring (zenithal and azimuthal) is strong, it fixes the orientation of the molecules close to the plates, and keeps them almost parallel to the plates while the field is applied. When the field is switched off, these fixed molecules reorient the other molecules according to the equilibrium texture. The elasticity of the liquid crystal, associated with the strong anchoring of molecules onto the plates, makes the deformation created by the field, and therefore all information, disappear.
A large amount of work has been done to improve the performances of these classical displays by optimising the physical properties of the liquid crystal including the temperature range, viscosity, elasticity, birefringence, dielectric anisotropy, Fréedericksz threshold voltage, etc. It is almost impossible for a pure product to satisfy all requirements. This is why mixtures comprising more than about ten components are generally used in display devices to optimise all “volume” properties of the liquid crystal [Handbook of Liquid Crystals (1998) Wiley-VCH Weinheim].
Furthermore, for these classical devices, the anchoring condition is not very strict, all that is necessary is that both anchorings should be “strong”, in other words larger than a limiting value (Laz<250 nm and Lz<15 nm). Research has been done on this problem in obtaining strong anchoring, but it can be considered to be almost solved at the moment. Several known polymer families arranged in the surface layer on plates provide strong anchoring adapted to classical displays [Liquid Crystals—Applications and Uses (1990) World Scientific Publishing Co. Pte. Ltd Singapore].
The strong azimuthal anchoring is obtained industrially by brushing the polymer film covering the plate. On most brushed polymers, the zenithal anchoring is one or two order of magnitude higher than the azimuthal anchoring. So classical LC displays have both strong azimuthal and zenithal anchoring.
3) Bistable Nematic Displays
A new generation of nematic displays called “bistable” nematic displays appeared a few years ago; they operate by switching between two stable states in the absence of a field. The external electric field is only applied for the time necessary to switch the texture of the liquid crystal from one state to the other. If there is no electric control signal, the display remains in its existing state. Due to its operating principle, this type of display consumes an amount of energy proportional to the number of image changes; thus when the frequency of image changes drops, the power necessary for operation of the display tends towards zero. This type of display is developing quickly due to expansion of the market for mobile devices.
For example, ZBD display uses a grating on one of the plate giving a bistable anchoring (G. P. Bryan-Brown et al., Nature, 399, 338 (1999)). With such a surface, the director of the liquid crystal molecules has two possible stable configurations with different pretilt angles, one high and one low, in the same azimutal plane.
4) Bistable Displays with Anchoring Breaking
Two bistable nematic displays with anchoring breaking using bistable surfaces have been proposed by the Orsay Solid State Physics Laboratory, one selects the stable state after switching by a flexoelectric effect (patent application FR 90 07847), and the other by an electro-chiral effect (patent application FR 90 01066).
Two bistable nematic displays with anchoring breaking on monostable surfaces are being developed at the moment: The BINEM® display developed by the NEMOPTIC Company in France (patent applications FR 95 13201 and FR 96 04447 and U.S. Pat. No. 6,327,017) or the SBiND display developed by the LICET Company in Italy (patent application EP 0 773 468 and U.S. Pat. No. 5,995,173 and patent application JP 9 274 205).
5) Description of the Bistable Liquid Crystal Device with Anchoring Breaking Called BINEM®
The bistable Binem device comprises a liquid crystal layer being situated between two substrate, forming a cell, one substrate called “master plate”, the other one called “slave plate”. Electrodes positioned on the inner side of the substrates allow electrical command signals to be applied to the liquid crystal layer situated between them for switching from one stable texture to the other. On the electrodes, alignment layers (for example polymer films) are deposited. The anchoring force is a characteristic of the interaction between the liquid crystal and the alignment layer. On the master plate of a BiNem device, the molecule zenithal anchoring is strong, on the slave plate the molecule zenithal anchoring is weak. The anchoring of the molecules on theses surfaces is monostable (FIG. 2), meaning that there is an unique alignment direction.
Switching of the BINEM® bistable display by anchoring breaking is diagrammatically shown in FIG. 2; it uses two textures, one uniform or slightly twisted texture T0 in which the molecules are approximately parallel to each other, and the other T180 that differs from the first by a twist of +/−180°. The nematic is chiralised with a spontaneous pitch p0, chosen to be close to four times the thickness of the cell to equalise the energies of the two textures. In the absence of a field, these are minimum energy states; the cell is bistable. In the presence of a strong field, an almost homeotropic texture (H) is obtained, and zenithal anchoring of the molecules is broken on at least one of the plates; neighbouring molecules are normal to it. At the end of the control pulse, the cell returns to one of the two textures depending on the rate at which molecules close to the surface, for which anchoring is not broken, return to an equilibrium. A slow return will lead to state T0 by elastic coupling between molecules close to the two surfaces, and a fast return will lead to state T180° by hydrodynamic coupling.
6) Characterisation of Anchoring Breaking
The orientation of liquid crystal molecules may be modified by external, electrical or magnetic fields. For example, for an electric field normal to the surface, positive anisotropic molecules are oriented along the field (θ=0°) in the body of a cell in which they would be flat (θ≅90°) in the absence of a field. On the surface, the director zenithal angle reduces continuously as a function of the field, and θ becomes zero if the field exceeds the critical field Ec. It is said that the anchoring is broken, since the director close to the surface is no longer affected by an anchoring torque or an electric torque. The critical field is:
                              E          C                =                              W            z                                                              K                33                            ⁢              Δ              ⁢                                                          ⁢              ɛ                                                          (        2        )            where Wz is the zenithal anchoring energy, K33 is the bending elasticity factor and Δ∈ the dielectric anisotropy.
This critical field is the field that must be applied to switch anchoring breaking devices. It is also the order of magnitude of the field necessary to change the orientation of the director close to surfaces in the case of devices in which breaking is not necessary. Mixtures with a high value of Δ∈ and high elasticity but a weak anchoring energy are necessary to obtain a device control field compatible with the control electronics.
In practice, the useful magnitude in the case of anchoring breaking displays is the voltage necessary to break them, therefore the critical field multiplied by the thickness of the cell. Cells of bistable displays are usually thick enough so that their birefringence is equal to the half wavelength of light at the centre of their passband. We will characterise zenithal anchoring using the voltage Uλ/2 at the terminals of a birefringence cell λ/2 for the yellow sodium line, when the field close to the surface is equal to the critical anchoring breaking field:
                              U                      λ            /            2                          =                                            λ              ⁢                                                          ⁢                              W                z                                                    2              ⁢                                                          ⁢              Δ              ⁢                                                          ⁢              n              ⁢                                                                    K                    33                                    ⁢                  Δ                  ⁢                                                                          ⁢                  ɛ                                                              =                                    λ                              2                ⁢                                                                  ⁢                Δ                ⁢                                                                  ⁢                                  nL                  z                                                      ⁢                                                            K                  33                                                  Δ                  ⁢                                                                          ⁢                  ɛ                                                                                        (        3        )            
In this relation, λ=589 nm is the wavelength of sodium, Wz, Lz, Δn, K33 and Δ∈ are parameters of the liquid crystal mixture, in particular its zenithal anchoring energy, its zenithal anchoring extrapolation length, its birefringence for the sodium line, its bending elasticity constant and its dielectric anisotropy, respectively.
The inventors consider that zenithal anchoring is weak when the breaking voltage Uλ/2 can be supplied, within the temperature range, by inexpensive electronic circuits. In practice, this can be represented by the empirical rule stating that anchoring is weak if Uλ/2<25 volts.
7) Nematic Mixtures for Displays with Anchoring Breaking
Displays with anchoring breaking that we have just mentioned and in general all bistable displays using switching on the surface, require weak and well-defined zenithal anchoring of the liquid crystal on it. Molecules on plates can then be aligned under the action of a reasonable breaking field compatible with the control circuits. Remember that the breaking field that aligns molecules attached to the surfaces is obviously greater than the field corresponding to the Fréedericksz threshold voltage which simply rotates molecules at the centre of the cells, and has almost no influence on the orientation of molecules close to the plates. Operation of anchoring breaking displays depends very closely on the “surface” properties of the nematic mixture. These physical properties (angles and anchoring force) characterise interaction of the liquid crystal with the alignment substrate.
Liquid crystals used by promoters of bistable nematic displays often belong to the cyanobiphenyls family, and they cannot be used to make devices operating within a wide temperature range. Furthermore, weak anchoring is often obtained on plates covered by mineral materials deposited by evaporation, this process being much more expensive than the usual polymer treatments. Normal materials and industrial surface treatments used for classical displays result in very strong anchoring and therefore cannot be used directly for bistable displays.
Thus, the production of bistable displays for practical, industrial or general public applications requires liquid crystal mixtures with a new property, namely weak zenithal anchoring. They also need to maintain more classical but essential volume properties; they must remain nematic within a wide temperature range and their mechanical, electrical and optical properties must be adjustable to satisfy the needs of the different bistable displays.
a) The Necessary Bulk Properties
The following gives a summary description of the volume properties of liquid crystal materials adapted to bistable displays.
In general, pure products are nematic within a restricted temperature range. The maximum value of this temperature range ΔTN is limited by TN-1, the nematic liquid crystal ⇄ isotropic liquid transition temperature, and the minimum value is limited by TX-N, the transition temperature towards more ordered liquid crystal phases or vitreous or crystalline solid phases.ΔTN=TN-1−TX-N
A wide temperature range is necessary for most applications, typically of the order of 50° C. to 80° C. centred on the ambient temperature (20° C.). In order to optimise the temperature range, the liquid crystal materials used must be composed of several components chosen from among chemical families with nematic ranges sufficiently offset in temperature and for which the distribution of molecular lengths hinders crystallisation. In practice, the number of components may be as high as a dozen to adjust the various mixture volume properties, while giving the required temperature range.
A good optical contrast of the display requires clearly defined birefringence of the mixture. The cell thickness for bistable displays with anchoring breaking is small; strongly birefringent mixtures are necessary (Δn=0.15 to 0.2). The mixture must include a high proportion of very anisotropic materials. The value of birefringence is critical but its adjustment is easy since birefringences of the different components of the mixture are added in proportion to their concentrations.
The display switching time is proportional to the viscosity of the material and is inversely proportional to its elasticity. In mixtures, it is known that highly anisotropic materials give high elastic constants, and also a high viscosity. The viscosity of the mixture may be reduced by slightly anisotropic additives with low viscosity that hinder the dimerisation of anisotropic materials. This will be essential to obtain response times of a few milliseconds necessary for applications, at low temperature.
Thus, nematic liquid crystals for displays with anchoring breaking should be mixtures composed of several mesomorphic materials; this is the only known method of obtaining the different volume properties necessary. Their diversity must not hinder the essential new property; weak zenithal anchoring. Furthermore for some applications, and particularly in the case of materials for BINEM® displays, it will be necessary to add a chiral additive to the mixture to obtain an infinite bistability.
b) Criterion of the Influence of the Liquid Crystal Material on the Anchor
The zenithal anchoring energy depends on treatments and the nature of the surface, the temperature and the liquid crystal material used. The inventors studied the influence of the material on zenithal anchoring, eliminating the influence of other parameters as far as possible.
The surfaces used in practice in liquid crystal cells also provide a non-zero azimuth anchoring. Most applications require that the azimuth direction of molecules on each plate is well defined so that the texture has the required optical properties. This azimuth anchoring is obtained industrially by brushing the polymer film covering the plate. The inventors discovered that the influence of brushing on the zenithal anchoring can be neglected as a first approximation.
The nature of the surface can make the zenithal anchoring energy vary by an order of magnitude. NEMOPTIC has developed polyvinyl chloride copolymer films (patent application FR 00 16135) on which the zenithal anchoring of pentyl-cyanobiphenyl (5CB) is weak (Lz>25 nm at +20° C.). On the other hand, polyimides used in classical displays give strong zenithal anchoring. For example, on a commercial polyimide orientation film (Nissan SE140), zenithal anchoring of 5CB is very strong (Lz≅7 nm to +20° C.). However the inventors have demonstrated that usually the surfaces that give the weakest anchoring for one liquid crystal will also give the weakest anchoring for many other liquid crystals. The inventors chose two surfaces with very different natures as being standard surfaces and they tested the zenithal anchoring energy of the liquid crystals studied for these two surfaces, to minimise the effect of the nature of the surface. These surfaces were chosen from surfaces for which the anchoring energy of 5CB is weak; a film of silicon monoxide (SiO) evaporated obliquely under vacuum and a brushed polymer film.
The SiO film is obtained by evaporation at 75°, it is 6 nm thick and at 20° C. it provides weak zenithal anchoring of the 5CB (Lz=31.3 nm which gives Uλ/2=14 volts according to equation (3)).
The brushed polymer film is denoted BP11, it is copolymer film of polyvinyl chloride prepared using the process described in patent application FR 00 16135. It provides weak zenithal anchoring of 5CB at 20° C. (Lz=27.5 nm, therefore Uλ/2=16 volts).
When the temperature decreases, the anchoring energy increases for all surface-liquid crystal pairs. This behaviour is explained by the reinforcement of the nematic order at low temperature, this order being transmitted close to the surface. The graph in FIG. 3 shows the critical breaking field Ec of the 5CB nematic on our two reference surfaces as a function of the birefringence Δn that is proportional to the order parameter (S). It is observed that the breaking thresholds depend more strongly on the nematic order on the polymer than on the silicon monoxide. The value of the parameter α for the approximation EC˜Sα is equal to approximately 4 (4.36±0.15) for the brushed polymer and 2 (2.15±0.1) for silicon monoxide. To enable comparisons, measurements will be made at the same reduced temperature Tred=(TN-1−T)/TN-1=0.9 (T is the Kelvin measurement temperature, and TN-1 is the temperature of the isotropic nematic transition).
The inventors chose the value of the breaking voltage Uλ/2 at a reduced temperature 0.9 on the two surfaces that they took as being standard surfaces, as a criterion for comparison of the zenithal anchoring of a material. In the following, zenithal anchoring will be considered as being weak when Uλ/2 is less than 25 V.
Limitation of Commercial Mixtures for Bistable Displays Using Anchoring Breaking
Using the criterion defined above, the inventors demonstrated that most commercial liquid crystal mixtures have strong anchoring; therefore they cannot be used to make bistable nematic displays with anchoring breaking.
Table 1 shows the critical voltage and the extrapolation length for different commercial products (MERCK) on a silicon monoxide film. Measurements were made at reduced temperature Tred=0.9.
TABLE 1Breaking voltage for λ/2Productcells: Uλ/2/voltsExtrapolation length Lz/nmMLC-665035.510.7MLC-6806-10037.716.5ZLI-108342.915.5ZLI-113251.013MLC-6809-00053.514.2ZLI-111560.713MLC-668660.815MLC-6846-00073.014MLC-482284.59.7MLC-6848-10086.07.6MLC-13300-00090.810MLC-6846-10095.28.2MLC-6012104.710.3MLC-6848-000108.27.2MLC-6625>119<14MLC-6849-100>1207.0MLC-12000-100>137<10MLC-50512137.4
It can be seen that anchoring of all these mixtures is strong despite their strong dielectric anisotropy since their critical breaking voltage Uλ/2 is greater than 30 volts.